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Between the glomerulus and Bowman's space there is a hydrostatic pressure gradient.
This gradient is normally the result of a LARGE glomerular hydrostatic pressure and a LOW pressure in Bowman's space which normally favors filtration.
Diagram here: http://physiologyplus.com/wp-content/uploads/2017/08/Glomerular-Filtration.png
In the case of post-renal obstruction, hydrostatic pressure behind the blockade will rise and urine will reflux into the capsular space and renal tubules (while glomerular hydrostatic pressure is unaffected) effectively decreasing the pressure gradient which reduces the filtration rate.